We measured the steady state and time resolved emission intensity decay of horse heart myoglobin at various pH values from neutral pH to pH 4.42. The steady-state intensity increased with decreasing pH. At pH 4.5 it was about double that at neutral pH. The phenomenon started around pH 5.0 and was entirely reversible from pH 4.5. Frequency domain data for emission decay were analyzed separately for each pH and simultaneously by global analyses. The results indicated the presence in the system of four lifetime components, whose length was conserved throughout the pH titrations at 40, 116, 1363 and 4822 ps, respectively. The titration affected only the amplitudes of the lifetime components thereby proving that they were not due to impurities. Assignments of the various lifetimes were based on the Forster theory of radiationless dipole-dipole interaction between tryptophan and heme, and the atomic coordinates of the system. We assigned the two shorter lifetimes to TRP-14 and TRP-7, respectively, in the presence of normally positioned hemes. The 1363 ps lifetime was assigned to TRP-7 in the presence of inverted hemes (i.e., rotated 180x around the `-~-meso axis of the porphyrin ring). The 4822 ns lifetime was assigned to reversibly dissociated heme-free myoglobin. Lorentzian lifetime distributions were narrow for the lifetimes at 40,116, and 4822 ps, indicating the presence of a homogeneous protein structure. Instead the lifetime at 1363 had a broad, pH independent distribution consistent with the presence of a homogeneous protein structure.